Code coverage with jake

This is an alternative option to #17 that uses [jake][] instead of `grunt` or `make` to handle the test coverage build.

`jake` is based on [rake][], which I've used and I like better than `make` as a build tool. I think it strikes a nice balance between providing a clear task definition structure and some basic helpers, without burying more sophisticated build processes under plugins and verbose configuration, which seems to be something some people don't like about `grunt`. `jake` provides some nice helpers on top of `exec` in particular that made the test coverage task easier to implement.

I don't plan to try out any more build tools. I'd suggest we try living with #17 or this PR for a while and see what happens.

[jake]: https://github.com/mde/jake
[rake]: http://rake.rubyforge.org/